The present invention relates to a forging method, more specifically a forging method that improves workability in machining by turning the metallographical structure of products subject to impact load to a fine ferrite-perlite structure, without adopting the method of quenching and tempering, to obtain as strength a yield point (YP value) exceeding that by the method of quenching and tempering, and making the tensile strength (TS) smaller than that obtained by the method of quenching and tempering.
Conventionally, products subject to impact load such as connecting rod, steering knuckle, crankshaft, etc., for example, used to be manufactured by forging.
Further, for the manufacturing of a connecting rod, which is momentarily subject to a large impact load, the method of quenching and tempering was also used in combination with forging, to increase its strength.
However, this method of quenching and tempering not only requires a high manufacturing cost but also is unfit for products mass-produced at low cost like automobile parts, for example, today when reduction of manufacturing cost is strongly called for. For that reason, a non-refining method capable of reducing manufacturing cost is coming to be adopted in place of the method of quenching and tempering.
This non-refining method consists of forcibly air cooling, after forging, high-temperature products from around 1200xc2x0 C. immediately to around 500xc2x0 C.
By the way, with the non-refining method by which high-temperature products at around 1200xc2x0 C. are forcibly air cooled, after forging, immediately to around 500xc2x0 C., the yield point (YP value) drops although the tensile strength (TS) remains at about the same level as with the method of quenching and tempering, and its value expressed by dividing the yield point by the tensile strength, i.e. value expressed in yield ratio (YR) is approximately 0.6. For that reason, this drop of yield point (YP value) as compared with the method of quenching and tempering puts an obstacle to reduction of weight of forged projects, while on the other hand a high tensile strength (TS) still remaining at about the same level as in the method of quenching and tempering means poor workability in machining in the same way as products manufactured by the method of quenching and tempering, and such were problems with the non-refining method.
In view of the problems with conventional forging methods, the objective of the present invention is to provide a forging method that improves workability in machining by turning the metallographical structure of products subject to impact load into a fine ferrite-perlite structure, without adopting the quenching and tempering method, to obtain, as strength, a yield point (YP value) exceeding that obtained by the quenching and tempering method, and reducing the tensile strength (TS) compared to the quenching and tempering method.
To achieve said objective, the forging method according to the present invention is characterized in that a forged material is manufactured by adding at least one kind of group metal, (or group VB metals), to a material to be forged, heating to a temperature suitable for hot forging, forging to a prescribed shape, cooling, holding for a prescribed set time in a furnace at a tempering temperature, and then further cooling to normal temperature by natural cooling.
In accordance with the present invention, it is desirable to set the xe2x80x9ctempering temperaturexe2x80x9d at a temperature in the range of 500xcx9c700xc2x0 C. and the xe2x80x9cprescribed set timexe2x80x9d for 30xcx9c60 minutes.
In this forging method, a forged material manufactured by adding at least one kind of group 5 metal to metal material consisting of perlite, ferrite, etc., which are usually used as forged materials, is heated to a temperature suitable for hot forging, is forged to a prescribed shape, is cooled, is then held for a prescribed set time in a furnace at a tempering temperature, and is then further cooled to normal temperature by natural cooling. Accordingly, group 5 metals such as vanadium, niobium, etc. that were added to the forged material can precipitate, on ferrite, fine carbon nitride mainly comprised of added elements, and thereby enable the setting of a high yield point (YP value) with high rigidity and strong resistance to impact load because of the fine metallographical structure of fine ferrite+perlite. The present invention therefore makes possible to reduce the weight of forged products, control a low tensile strength (TS), and thanks to the fine metallographical structure of fine ferrite+perlite, improve workability in machining, thus promoting the reduction of manufacturing costs for forged products.
In accordance with the present invention, the heating temperature of the forged material shall preferably be set in the range of 1150xcx9c1250xc2x0 C.
This promotes melting into a solid solution of group 5 metals such as vanadium, niobium, etc. that were added to the forged material. When they have cooled and precipitated, the texture of the forged material is strained with the precipitate, and precipitates as a large volume of fine carbon nitride, while the strength of the forged material increases because the metallographical structure becomes fine.